Description of the Conventional Art
Conventional nonvolatile memory devices retain stored data even when powered off. Examples of conventional nonvolatile memory devices are ferroelectric memory devices, magnetic memory devices and phase change memory devices.
A unit cell of the phase change memory device uses a phase change material to store data. A phase change material changes into one of two stable sates (e.g., an amorphous state and a crystalline state), depending on the temperature and duration of supplied heat. A phase change material in an amorphous state has a higher resistivity than the phase change material in a crystalline state. The amount of current flowing through the phase change material differs according to a difference between the resistivity of the phase change material in the amorphous state and the resistivity of the phase change material in the crystalline state. Whether the stored data is a logical “1” or “0” is determined using the difference in the amount of current.
FIGS. 1 and 2 are sectional views illustrating a conventional method for fabricating a conventional phase change memory device.
Referring to FIG. 1, an interlayer oxide layer 2 is formed on a semiconductor substrate 1, and the interlayer oxide layer 2 is patterned to form a contact hole. A first conductive layer, filling the contact hole, is formed on the semiconductor substrate 1. A contact plug 3 is formed by planarizing the first conductive layer until the interlayer oxide layer 2 is exposed. A phase change layer 4 and a second conductive layer 5 are sequentially formed on the semiconductor substrate 1 including the contact plug. A mask pattern 6 covering the top surface of the contact plug 3 is formed on the second conductive layer 5.
Referring to FIG. 2, using the mask pattern 6 as an etch mask, the second conductive layer 5 and the phase change layer 4 are sequentially patterned to form a phase change pattern 4a and a conductive pattern 5a that are stacked sequentially. The phase change pattern 4a contacts the contact plug 3. At least a portion of the phase change pattern 41 corresponds to a program region 8. The program region 8 changes into an amorphous state or a crystalline state, depending on the temperature and supply duration of Joule heat generated by the contact plug 3.
In the conventional phase change memory device shown in FIG. 2, a region 7 adjacent to the side surface of the phase change pattern 4a may be damaged during an etching process using the mask pattern 6, which may change and/or degrade phase change characteristics of the region 7. The program region 8 may have a hemispheric structure surrounding the contact surface between the phase change pattern 4a and the contact plug 3. In this example, a portion of the program region 8 may belong to the damaged region 7, which may degrade characteristics of the program region 8 and lead to malfunction of the phase change memory device. As the integration degree of semiconductor devices increases, the size of the phase change pattern 4a decreases, which may emphasize and/or worsen the defects of the phase change memory device due to the damaged region 7.